Speaking to MEPs in the European Parliament, Barnier regretted that Johnson's new government had expressed its intention to "reverse the commitments" on trade made by his predecessor Prime Minister Theresa May.

These pledges were part of a non-binding "political declaration" on the future relationship between the EU and UK that stands beside a highly contentious withdrawal deal that has failed to win approval in the British parliament.

In addition to rejecting the divorce deal, Johnson's negotiators have asked that Britain be allowed to widely diverge from EU rules after Brexit, all while securing a highly ambitious trade deal with Europe.

Barnier on Wednesday poured cold water on that British request, insisting that a comprehensive trade accord could only come by closely abiding by EU rules.

"The level of ambition of a future free trade agreement that we will have to negotiate with the United Kingdom will clearly depend on the guarantees that we have agreed in the social, environmental, competition or state aid fields," Barnier said in Strasbourg.

"This economic relationship must obviously be accompanied by guarantees of a level playing field," he added, referring to a commitment by trading partners to abide by the same norms.

A senior European official told AFP that the EU is not ready to accept Britain's requests.

The official believed Johnson wants the UK to become a "Singapore" of Europe, where multi-nationals can flout EU rules but keep undisturbed access to Europe's market of 500 million consumers.

"With a large country so close and important like the United Kingdom, which does just over half of its trade with the (EU) single market, an ambitious economic partnership requires a set of common rules of the game," Barnier said.

London and Brussels are struggling to agree on changes to the withdrawal agreement, with Britain on course to leave the EU on October 31.

"If the United Kingdom comes out without an agreement, I want to remind you that all these issues do not disappear," Barnier added.